The growing demands for concrete in building and construction applications require increased volumes of Portland cement, which is costly. To reduce the amount of Portland cement needed to make concrete, substitute compounds and compositions are needed.
Fly ash is produced from burning pulverized coal in a coal-fired boiler. It is a fine-grained, powdery particulate material that is carried off in the flue gas and may be collected from the gas by electrostatic precipitators or mechanical collection devices, including cyclones. Fly ash is typically disposed of in landfill areas.
Fly ash may be used as an admixture to Portland cement and as a component of a Portland-pozzolan blended cement. When used in concrete, fly ash should have sufficient pozzolanic reactivity and should be of consistent quality. Mixtures of aggregates and binders present in Portland cement, for example, increase the strength, bearing capacity and durability of a structure. Fly ash exhibiting pozzolanic or self-cementing properties may be used as an additive to concrete, and may replace a portion of the cement needed in forming concrete. In addition to the cost savings, the addition of fly ash also improves the properties of concrete, for example, reduced permeability and improved workability.
To be used in Portland cement concrete, fly ash must meet the requirements of the American Society for Testing and Materials, ASTM C-618, which includes Classes N, F and C. Class F, for example, has a carbon content of from 4.8 to 12 percent.
The chemical and physical properties of fly ash are influenced by those of the coal burned and the techniques used for handling. The properties are also influenced by the individual combustion techniques and associated efficiency of the individual boilers.
The four types of coal are anthracite, bituminous, subbituminous and lignite. A principal component of fly ash is determined by the loss on ignition (LOI). LOI is the measurement of the amount of unburned carbon remaining in the fly ash, which indicates the suitability for use as a cement replacement in concrete. Generally, if the LOI is more than 6 percent, the fly ash does not meet ASTM standards and cannot be used for concrete. For example, fly ash which does not meet an individual state Department of Transportation requirement for concrete (generally corresponding to ASTM C-618 or AASHTO 295), is considered “off grade” in that the carbon exceeds, for example, the Ohio, Kentucky, and Indiana state maximum allowable LOI of 3%. Fly ash available from Beckjord Generating Station, a power plant in New Richmond, Ohio, is an example of off-grade fly ash for concrete products. Other variables, including fineness and variability, are other factors which cause a particular fly ash to fail the ASTM or the American Association of State Highway Transportation Officials (AASHTO) requirements.
Air entrainment is a factor for durability in concrete to resist freezing and thawing conditions in hydraulic cementitious compositions, for example, mortar, masonry and concrete. The presence of entrained air therefore is significant for the long-term durability of concrete or mortar. Entrained air is characterized by substantially uniformly disperse, spherical spaces in cement paste, whereas entrapped air is characterized by irregularly shaped voids which are not generally uniform in size, but are larger than “entrained” air voids. The carbon present in fly ash (and the high carbon content in off-grade fly ash) may adsorb surfactants or “air-entraining” admixtures used in making concrete, thus rendering the surfactants or admixtures unavailable for the intended purpose, and also reducing air entrainment of the cement mixtures. Residual carbon in ash may also interfere with the air entrainment process in forming concrete.
The presence of fly ash in cement mixtures or concrete has several advantages, including the reduction of fly ash disposed in landfill, thus protecting natural resources, lower cost than other additives, and it decreases permeability and shrinkage in the hardened concrete. Other advantages include durability and long-term strength gain, in that there is reduced dry shrinkage, reduced heat of hydration, water reduction, reduced alkali silica reactivity, and increased resistance to sulfate attack. In addition, the workability is improved, in that there is reduced bleeding and segregation, improved flowability, and improved finishing characteristics. Additionally, there is the reduction of carbon dioxide by the replacement of a portion of the cement.
A need remains for a compound, composition and method for treating fly ash for use in cementitious compositions.